With a transmission of PET detector signals to the evaluation unit, when the positron emission tomography (PET) system is integrated into magnetic resonance (MR) tomography, the magnetic and electrical fields of the MR gradient system influence the analog signal processing electronics of the PET system and can induce interference signals, particularly at sensitive points (e.g. photodetectors and preamplifiers). In addition to the low-frequency gradient fields (frequency range 0 . . . 10 kHz) required for operation of the MR, there are often interference frequencies emanating from the gradient amplifier in the range of a few 100 kHz, which occur in the normally switched-mode operated power end stages and can often be adequately suppressed only with difficulty by filters in the gradient supply lines. The particular problem in this case is the spectral overlapping with the parts of the PET signal spectrum (under 5 MHz) which are necessary for a high accuracy when measuring the energy of the PET events. This energy resolution is important on the one hand for the qualification of pulse pairs compared with scattered quanta and on the other hand for the Anger method for lateral localization of the ionization within a scintillator block.
Therefore, the transmission of sensor signals with as little interference as possible is very important.
From DE 10 2006 027 417 A1, a sensor device is known which is particularly intended for a PET detector, which is operated in a magnetic field, which varies over time, of a magnetic resonance tomography system. It includes a sensor circuit for generating a sensor signal and an induction circuit in which a compensation signal is induced. These signals are combined in such a way that interference signals in the sensor signal, induced by the magnetic field which changes over time in the sensor circuit, are compensated for.
Design measures such as the minimization of the induction areas spanned by the signal lines or electrostatic screening can in fact reduce the interference signal injection. However a complete elimination of the injection is frequently not possible because of the difficult boundary conditions (e.g. temperature rise, vibration and secondary gradient fields due to eddy currents on screening surfaces).